1. Field of the Invention
This invention relates to video data, and more particularly to a picture image processor which may attain shading correction and binary conditioning for signals without using a high-speed A/D converter or arithmetic circuit, and which may also correct shading distortion without using an A/D converter having a large number of bits, and which may also obtain an exact multiple gradient output even if there is a high dark voltage for an image sensor.
2. Description of the Prior Art
In an image reader wherein a fluorescent lamp or the like is utilized as a light source, the surface of an original is irradiated with the light, and the reflected light is read by an image sensor through lenses; generally there is a nonuniformity of the luminous energy in the light receiving section of the image sensor due to differences in luminous conditions of the respective portions of the fluorescent tube or differences in transmittance of the respective portions of the lenses, and with the passage of time, a temperature rise of the image sensor brings about a ununiform output condition.
Recently, a solid image sensor composed of a CCD device and an optical sensor has been proposed. Such a sensor, however, inevitably has variations in the light-receiving sensitivity of each element, because the sensor is constructed by integrating an array consisting of a number of light receiving elements combined with a CCD multiplexer. Identification of an object to be picked up is significantly influenced by differences in sensitivity between such elements in the case where a difference in level between the object and background noise component is slight, so that there is a case where correct picture image reproduction is damaged.
In recent years, conventional correction for such nonuniformity in luminous energy or sensor sensitivity has been widely effected after the sensor output had been digitized. Most shading correction circuits operate by storing a correction value which has been obtained by first reading a white reference sheet or the like into a RAM, and then reading the correction value simultaneously with the input of a digital signal to be corrected, which is obtained by reading an original, and then inputting the information to the arithmetic circuit.
In this respect, however, with the progress of technology, the frequency of the processing digital signals to be corrected becomes high so that a speeding up of the dynamic sensitivity has been required.
As an example of a picture image processor wherein shading correction is effected by digital processing, there is the processor disclosed in Japanese Laid-open application No. 64867/1983 in which a prescribed amount of attenuation is applied to an output of the detection element of an image sensor to equalize the respective outputs, and the amount of attenuation is stored in a memory; another processor is disclosed in Japanese Laid-open application No. 153377/1984 wherein an original reading signal and a shading correction signal are stored in the memory as addresses.
In these conventional picture image processor, however, complicated arithmetic circuits, high-speed A/D converters and the like are needed to attain a speeding up thereof, so that such processor have a disadvantage due to their high cost.
As still another example of a conventional image processor, there has been provided a processor wherein an analog signal to be used as a reference is digitized by an A/D converter; then, the digitalized signal stored in a memory (RAM), and thereafter, an output signal of the image sensor is synchronized with a memory reference signal for each picture element of a line sensor, whereby a corrected output Z is obtained by means of a digital computing element through the operation of the following equation: EQU Z=X/Y.multidot.C (1)
where X is a sensor output, Y is a reference signal, and C is a coefficient.
In the above construction, distortions in the radiant values of the picture image picked up by the sensor, e.g., shading distortion produced from limb darkening derived from the optical system, or irregularity in response characteristics of sensor system and the like have previously been stored in the memory as a reference signal. As a result, when the operation of equation (1) is executed by means of the digital computing element while synchronizing such a reference signal with output signal of the sensor, an output signal Z, whose shading distortion has been corrected, can be obtained. In this case, if the converted precision of the A/D converter is n bits, then the numeric value expressed by the digital operation becomes 2.sup.n.
Furthermore, there is a method of correction of shading distortion disclosed in Japanese Laid-open application No. 175279/1984 wherein the correction is attained by comparing exact shading distortion which has been previously stored with picture image reading signal.
However, in the above described conventional picture image processor, since a range of expressible values which correspond to variations and which are contained in the reference signal is restricted (for example, when a variation in shading is assumed to be .+-.2.sub.i, then the expressible value is 2.sup.n-2i.), in other words, since the effective bit number is restricted to n-2.sub.i bits, it is necessary to use an A/D converter having several more bits than the effective bit number in order to correct shading distortion variations, so that such a processor has a disadvantage due to its high cost.
Moreover, the output voltage of an image sensor used in a picture image processor varies with variations in the luminous energy of the illumination light source to be applied to an original or changes in the ambient temperature and the like. Particularly, in a one-dimensional image sensor wherein a plurality of sensors are aligned along the principal scanning direction of an original, the output voltage does not become uniform due to difference in sensitivity of the sensors for each picture element, so that variations of luminous energy in the principal scanning direction of the illumination light source appear.
In this connection, there has been proposed a prior art picture image processor wherein the output signal of the image sensor is subjected to an analog-to-digital conversion, and then compression and expansion are conducted in a concentration region in response to differences from the reference voltage in order to correct such fluctuations or ununiformity in the output voltage of the image sensor.
In the prior art as described above, however, when an image sensor, having a fluctuating range of dark voltage which is comparatively wide so that the fluctuating range of the output voltage also becomes wide, such as a close contact type amorphous silicon image sensor or the like is utilized, there was a problem in that errors appear in compression or expansion due to the influence of the dark voltage in the concentration range, and as a result thereof, a multiple gradient output is not obtained in an exact manner.